Frequently Asked Question
Multiphase flow contains two or more interpenetrating materials or phases with distinct velocities, properties, or interfaces. The first decision is whether the phases form a resolved interface, a dispersed population, or an interpenetrating mixture at the scale of interest.
| Physical view | Useful when | Key quantities |
|---|---|---|
| Resolved interface | Interface shape and motion are central | Surface position, curvature, pressure jump |
| Dispersed phase | Particles, droplets, or bubbles are smaller than cells | Volume fraction, slip, drag, dispersion |
| Mixture | Phases are strongly coupled at the modeled scale | Mixture velocity, relative flux, phase fraction |
Model selection should follow the measurable output and available data, not simply the number of phases.
Phase interaction
Multiphase flow requires conservation of mass and momentum for each phase together with interfacial exchange. The important scales include volume fraction, interface or particle size, relative velocity, density and viscosity ratios, surface tension, residence time, and phase-change rate. The representation must match the desired output: a resolved free surface, an averaged mixture, or dispersed particle and bubble statistics.
α is volume fraction, We is the Weber number, ρ is density, U is relative speed, L is a characteristic size, and σ is surface tension.
Worked example
With ρ = 1000 kg/m³, U = 1 m/s, L = 0.005 m, and σ = 0.072 N/m, We = 69.4. Inertia is therefore significant relative to surface tension, so interface deformation should be investigated rather than assuming a spherical phase.
Check: assess phase conservation separately, resolution, exchange time scale, coalescence or breakup assumptions, and inlet phase-fraction sensitivity.